High crystalline/high tacticity (low xylene soluble levels less than 3%) polypropylene homopolymers, such as, Total 3270, Phillips 66 CH020XK, or Braskem Inspire 6025 are used as the core resin in several clear and cavitated multilayer, extruded biaxially oriented film structures where maximized stiffness and tensile strength are desired.
These high crystalline polypropylene homopolymers are very difficult to uniformly orient and/or avoid breakage during the orientation process. Compared to forming biaxially oriented film with conventional, low or medium crystalline polypropylene as the core layer; forming a biaxially oriented film with high crystalline polypropylene as the core layer requires higher machine direction orientation (MDO) and transverse direction orientation (TDO) stretch temperatures, and often requires lower line speeds to allow for sufficient heating in the ovens during the pre-draw and draw stages. High MDO stretch temperatures can be particularly problematic with film structures in which one or both skin layers comprise a low melting point copolymer as a primary component.
In order to reduce the stress to orient and/or the necessity to raise the temperatures to orient a cast sheet employing a high crystalline polypropylene homopolymer in the core layer of a multi-layer, biaxially oriented film, it has been suggested to utilize, or add, polypropylene copolymers and homopolymers to the high crystalline core as a processing aid. For example, both Basell Adsyl 7416 (copolymer with ˜5% ethylene, 7.5 dg/min MFR, 133° C. melting point) and Braskem DS6D21 (copolymer with ˜2.5% ethylene, 8 dg/min MFR, 146° C. melting point) ethylene propylene random copolymers have been used effectively to improve the processability and line rate efficiency in forming a multi-layer, biaxially oriented film employed as a base film for a metallized film structure and employing either Total 3270, Phillips 66 CH020XK, or Braskem Inspire 6025 (i.e., high crystalline polypropylene homopolymer) as the primary core resin. The inclusion of the polypropylene copolymers as a processing aid for a high crystalline polypropylene core layer reduces the stress to orient the film.
However, this reduction in stress to biaxially orient the film and the corresponding improvement in processability on a continuous tenter line comes at the expense of several key film properties; most notably tensile modulus and strength.
It also has been suggested to employ a high melt flow polypropylene homopolymer as a processing aid to reduce extrusion pressure below the maximum safe operating pressure, as disclosed in Ackermans et al. US publication 2014/0343230. Typical commercially available grades of metallocene-catalyzed polypropylene within the melt flow range described in Ackermans et al, and which are sold into the melt blown fiber industry are Metocene MF650W, Metocene MF650X and Metocene MF650Y; sold by Equistar Chemicals, L.P. (a wholly owned subsidiary of LyondellBasell Industries, Houston, Tex.). These commercial grades of metallocene-catalyzed homopolymers have not provided the desired degree of stress reduction in the film orientation process.
Crystalline polypropylene waxes, including crystalline metallocene catalyzed polypropylene waxes are known, but have not been recognized or known to be an effective processing aid in the extrusion formation of a film layer from predominately high crystalline polypropylene homopolymer.
Specifically polypropylene waxes are used industrially in many ways, e.g., as dispersants for pigments for coloring thermoplastic polymers, as auxiliaries in plastics processing, as matting and abrasion protection additives in printing inks and surface coatings, as constituents of photo toner compositions and in formulations for hot melt compositions. Many of these applications require high degrees of crystallinity and high melting points. For example, the heat resistance of hot melt compositions can be increased by use of polypropylene waxes having a high melting point. As matting and abrasion protection agents in printing inks and surface coatings, the waxes are used in milled, frequently also micronized, form. High degrees of crystallinity are advantageous here since these are associated with product hardness, which aids the milling process or is necessary to make the desired small particle size possible at all. In addition, high hardness produces an improved abrasion protection action.
However, none of the prior art known to applicant provides any teaching of the properties (e.g., crystallinity, melting point and viscosity) of crystalline polypropylene wax, e.g., crystalline metallocene catalyzed polypropylene wax, that are important, desired or required in order to effectively employ such waxes as a processing aid in accordance with this invention.
A need exists for processing aids for permitting the effective orientation of a film layer including high crystalline polypropylene homopolymer and preferably employed as a base or core layer, and or in one or more intermediate layers in a multilayer, coextruded, biaxially oriented film and that has advantages and/or benefits over prior art processing aids. The present invention is directed to such an improved processing aid for permitting the effective orientation of an extruded film layer; most preferably a film layer constituting a core layer and optionally one or more intermediate layers of a multilayer extruded film that is biaxially oriented, to an oriented film layer including a blend employing a processing aid for aiding in the effective orientation of the film layer, to a method of orienting the film layer and to a biaxially oriented, multilayer film employing the film layer as a core or base layer and/or one or more intermediate layers of such structure.
The invention described hereinafter employ cost effective processing aids that address the processability issues (e.g., uniform orientation of an extruded layer with minimal breakage) while minimizing the adverse effect on film properties such as tensile modulus and strength.